A composite component or structure, referred to hereinafter as simply composite structure, is a term generally used to describe any part consisting of at least two constituents that are combined yet retain their physical and chemical identities. One type of composite structure is a particulate reinforced composite (PRC) in which particulates of a selected material are embedded or bonded into a matrix. An advanced composite structure is a term generally used to describe fibers of high strength and modulus embedded in or bonded to a matrix, such as a resin, metal, ceramic, or carbonaceous matrix. The fibers may be continuous fibers, short fibers, or whiskers. The resin type matrix may be a polymerized synthetic or a chemically modified natural resin, which may include but is not limited to thermoplastic materials such as polyvinyl, polystyrene, and polyethylene and thermosetting materials such as polyesters, epoxies, and silicones. Typically, a distinct interface or boundary is present between the fibers and the matrix material. It is appreciated that the composite structure produces a combination of properties that cannot be achieved with either of the constituents acting alone.
The composite structure is typically produced by a multi-step process that begins with laying up the fibers generally in swatches of material known as laminates or plies on an impervious surface. To form the matrix about the fiber plies, the plies may be pre-impregnated with the matrix material or may be un-impregnated. The un-impregnated fibers may be embedded or bonded in the matrix material by using injection molding, reaction injection molding (RIM), resin infusion, or other matrix embedding or bonding techniques. Once the fiber plies are arranged in a desired configuration, compaction techniques such as vacuum bagging are advantageously employed to remove voids from the fiber plies. The matrix material surrounding the plies may be cured employing ovens, electron beams, ultraviolet, infrared light sources, autoclave cured. Curing may be carried out at room (i.e., ambient) or elevated temperatures.
One existing manufacturing process for producing large, complex-shaped, three-dimensional, fiber reinforced composite structures includes arranging fiber plies arranged on plaster mandrels to form the complex shape. Fiber reinforced plies are laid up and impregnated on the plaster mandrels, which have been previously varnished to seal them. The resulting structure is vaccuum bagged and cured in an autoclave. The plaster mandrel is removed by striking it through the laid up, crumbling the plaster mandrel to leave the hollow composite structure. This technique is commonly used to produce structures such as complex-shaped, air conditioning ducts. This type of tooling may include locking features that hold the tool's complex shape.
If the strength of the component is at issue, steel, aluminum, or invar tooling materials may be used to create shapes that can be fastened or otherwise coupled together to create a mold surface for laying up the fiber plies. For example, an auxiliary power unit inlet duct for an airplane typically requires structural materials that exceed the strength requirements obtainable from the plaster mandrel techniques described above.
Another method of producing large composite core structures formed by vacuum assisted resin transfer molding is described in U.S. Pat. No. 6,159,414 to Tunis, III et al. (Tunis). Tunis describes making composite structures by employing hollow cell or foam block cores. The cores may be wrapped with a fiber material and arranged in a mold such that the fiber material forms a face skin. The assembly is sealed under a vacuum bag to a mold surface. One or more main feeder conduits communicate with a resin distribution network of smaller channels, which facilitates flow of uncured resin into and through the fiber material. The resin distribution network may comprise a network of grooves formed in the surfaces or the cores and/or rounded corners of the cores. The network of smaller channels may also be provided between the vacuum bag and the fiber material, either integrally in the vacuum bag or via a separate distribution medium. Resin, introduced under vacuum, travels relatively quickly through the main feeder channel(s) and into the network of smaller channels. After penetrating the fiber material to reach the surface of the cores, the resin again travels relatively quickly along the cores via the grooves in the cores or the spaces provided by the rounded corners to penetrate the fiber material wrapped around and even between the cores. The resin is then cured in an autoclave after impregnating the fiber material to form a three-dimensional fiber reinforced composite structure.
The aforementioned technologies may have a propensity to buckle, warp or otherwise undesirably distort various structural features of the fiber-reinforced structure. For example, shear webs, plates and other planar surfaces that do not come into contact with the tool may buckle during the autoclave and cure process. The buckling of a shear web, for example, may distort the web into an undesired shape normal to its planar surface.